1. Field of the Invention
The present invention relates to differential amplifiers, and more particularly to differential amplifiers where the output voltage level is an issue with respect to following circuitry.
2. Background Information
Differential analog amplifiers carry information in the input difference signal. The average signal level of the inputs is usually not of interest. However, the average voltage level of the output (commonly called the common mode or CM level) may produce an output voltage level that is inconvenient for following electronics. This may force the circuit to be AC coupled using large capacitors, etc. Inconvenient output voltage levels also may reduce dynamic range. For example, in applications where the output of a differential amplifier drives a circuit referenced between a single supply source and ground, positioning the differential amplifier output level to about mid-range of the supply voltage is often beneficial, but may not be possible without additional level shifting circuitry.
Another limitation may occur when an amplifier is used with feedback. In such an application, the circuit is limited to a gain/bandwidth product where the dynamics of the input stage affects the performance and the stability of the circuit. For example, higher input stage gain may cause oscillations.
Current feedback operational amplifiers have a high slew rate and, typically, are not subject to the gain/bandwidth limitation of voltage operational amplifiers. The bandwidth of a traditional current feedback operational amplifier depends primarily on the value of the feed back resistor. Typically such amplifiers are used in high bandwidth amplifiers, and have characteristics that make it better suited for specific applications, e.g. video signal processing.
Current feedback operational amplifier circuits, however, have other limitations that may make them unsuitable for some applications. For example, they typically have a near zero impedance at the inverting input while the non-inverting input has a near infinite input impedance. Also, the current noise is higher but the voltage noise is lower in current mode operational amplifier circuits.
It would be advantageous to have differential operational amplifiers with high input impedances, that accommodate CM voltages, that independently set output voltage levels, and where gains may be set without changing bandwidths or otherwise affecting amplifier performance.